Method and apparatus for measuring particle concentration in a fluid suspension of particles

ABSTRACT

The apparatus includes (1) a Coulter type particle analyzing device for producing a particle pulse each time a particle is in a sensing zone, each particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, (2) electrical circuitry connected to the output of the particle analyzing device for producing, upon sensing each particle pulse above a predetermined threshold level, an output pulse having a selected clearly defined duration established between (a) a clearly defined leading edge of the output pulse occurring at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse and (b) a clearly defined trailing edge of the output pulse occurring at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and (3) apparatus for measuring and displaying the duty factor of a train of the output pulses. The method includes the steps of: initiating an output pulse for each particle pulse at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse, terminating each output pulse at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and measuring the duty factor of a train of the output pulses. This duty factor is closely proportional to particle concentration.

United States Faterrt [1 1 Coulter et al.

May 15,1973

[54] METHOD AND APPARATUS FOR MEASURING PARTICLE CQNCENTRATKON IN A FLUID SUSPENSION OF PARTKCLES [75] Inventors: Wallace H. Coulter, Miami Springs; Walter R. Hogg, Miami Lakes; Millard Dumas Longman, North Miami, all of Fla.

[73] Assignee: Coulter Electronics, Inc., Hialeah,

Fla.

[22] Filed: Jan. 4, 1972 [21] Appl. No.: 215,374

[52] US. Cl ..324/71CP, 235/92 PC, 307/261 [51] Int. Cl. ..Gln 27/100 [58] Field of Search ..324/71 CP;

235/92 PC; 73/432 PS; 250/218; 356/39, 102, 208; 307/261 [56] References Cited UNITED STATES PATENTS 3,259,842 7/1966 Coulter et al ..324/7l CP 3,441,848 4/1969 Valley et al ..324/7l CP Primary Examiner-Rudolph V. Rolinec Attorney- 1. Irving Silverman et al.

[57] ABSTRACT The apparatus includes 1) a Coulter type particle analyzing device for producing a particle pulse each time a particle is in a sensing zone, each particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, (2) electrical circuitry connected to the output of the particle analyzing device for producing, upon sensing each particle pulse above a predetermined threshold level, an output pulse having a selected clearly defined duration established between (a) a clearly defined leading edge of the output pulse occurring at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse and (b) a clearly defined trailing edge of the output pulse occurring at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and (3) apparatus for measuring and displaying the duty factor of a train of the output pulses.

The method includes the steps of: initiating an output pulse for each particle pulse at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse, terminating each output pulse at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and measuring the duty factor of a train of the output pulses. This duty factor is closely proportional to particle concentration.

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METHOD AND APPARATUS FOR lWEASUNG PARTICLE CONCENTRATION IN A FLUID SUSPENSION OF PARTICLES BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for measuring particle concentration in a fluid suspension utilizing a Coulter particle analyzing device which produces a signal, commonly referred to as a particle pulse, each time a particle in a given amount of fluid passes through a sensing zone. The duration of each particle pulse is related to the time the particle is in the sensing zone. By measuring the duration of each pulse and then summing the durations, one can obtain an indication of the time particles are in the sensing zone. Then, by comparing this time with the total time during which the durations are summed, that is, by determining the duty factor of the pulse train, one can obtain a measurement of the concentration of particles in the fluid.

Typically, the sensing zone is in the vicinity of an aperture situated between two bodies of liquid and fluid flow of particles through the aperture alters the impedance through the aperture. This change in impedance generates a signal a particle pulse which, because of several factors, principally the short length of the aperture, which is desirably kept short in order to achieve a favorable signal-to-noise ratio, has poorly defined leading and trailing edges. Since almost all of the particle pulses have poorly defined leading and trailing edges, it is difficult to make an accurate measurement of the concentration of particles by measuring the beginning and end of each pulse. Accordingly, it is desirable to provide some means for generating an output signal or pulse for each particle passing through the sensing zone with each output pulse having a clearly defined duration related to the duration of each particle in the sensing zone.

Heretofore, it has been proposedin United States patent application Ser. No. 138,096 filed on Apr. 28, 1971, by two of the inventors of the present application (Wallace H. Coulter and Walter R. Hogg) to utilize signals particle pulses from a particle sensing zone for relating or comparing (a) the duration of time occupied by particles in suspension while traversing the sensing zone to (b) the total time to obtain a measurement of the concentration of particles in the fluid, the concentration being proportional to the percentage of time particles are in the sensing zone. For this purpose, the prior application teaches the measuring of the duration of each particle pulse by sensing or measuring two ascertainable points on each particle pulse and then producing a duration measuring pulse having a constant amplitude and well-defined leading and trailing edges at the measured points on the particle pulse.

It is to be noted that an electrical circuit for providing a constant amplitude pulse when a particle is passing through a sensing zone is disclosed in US. Pat. No. 2,775,159 issued to J. C. Frommer on Dec. 25, 1956. In the Frommer patent, the sensing zone is relatively long such that the resulting pulses are long compared to their rise and fall times. Consequently, the shapes of the leading and trailing edges of each pulse have little effect on the measurement of the duration of each pulse.

As brought out in the aforementioned United States patent application, Ser. No. 138,096, the Frommer circuitry is not fully adaptable for use with a Coulter particle analyzing device which utilizes a sensing zone defined in part by an aperture with a very short axial length. The aforementioned application is concerned with this problem of a sensing zone with a short length and proposes several methods and circuits for measuring a first point on a particle pulse and initiating an output pulse at that point in time and then terminating the output pulse at a second point on the particle pulse so that each output'pulse has a selected, clearly defined duration related to the duration of a particle in the sensing zone. Specifically, the aforementioned application discloses three systems for ascertaining two points on a particle pulse and for initiating and terminating an output pulse at thosetwo points. Each of these output pulses is applied to an integrator which integrates or sums the output pulses for a predetermined time to provide an indication of the concentration of particles in the suspension which flows through the sensing zone.

In one system disclosed in the aforementioned patent application, the duration or interval of the output pulse is from the time of the peak of the particle pulse to a later time at some lower level on the trailing edge of the particle pulse. This output pulse is obtained by measuring the interval between the peak of the particle pulse as sensed by the zero crossing of the first derivative of the particle pulse and some fractional height on the trailing edge of the particle pulse as determined by the setting of an attenuator across a pulse stretcher.

In another system disclosed in the aforementioned patent application, the output pulse is initiated at a predetermined amplitude level such as the half amplitude level on the leading edge of the particle pulse and is terminated at the same level of amplitude on the trailing edge of the particle pulse. In this system, the duration or interval of the output pulse is obtained by storing the amplitude of the pulse in a pulse stretcher and the undistorted pulse in a delay line or the equivalent. Then the stretched pulse is attenuated to a fractional height of the amplitude of the particle pulse, for example, to the half amplitude, and compared with the stored undistorted pulse. When the instantaneous value of the leading edge of the undistorted pulse exceeds the half amplitude of the particle pulse (i.e., the amplitude of the stretched and attenuated pulse), the output pulse is initiated. Then, when the instantaneous value of the trailing edge of the undistorted pulse falls below the amplitude of the stretched and attenuated pulse, the output pulse is terminated. This-system works very well but is rather expensive in view of the analog delay system needed to delay the particle pulse.

In still another system disclosed in the aforementioned patent application, the duration or interval of the output pulse is between the point of inflection of the slope of the leading edge of the particle pulse and the last point of inflection of the slope of the trailing edge of the. particle pulse. The pulse width or interval is obtained by measuring the first and last zero crossings of the second derivative of the particle pulse. This system is also rather expensive in that it involves extensive electrical circuitry to provide the desired output pulse.

The present invention provides another method and another system or apparatus for measuring two points on a particle pulse, the apparatus being very effective and yet less expensive than the two systems just described above.

SUMMARY or THE INVENTION According to the present invention, there is provided an apparatus and method for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by a particle analyzing device or electrical system which produces for each particle sensed a particle pulse having poorly defined leading and trailing edges and a duration or interval related to the time the particle is in the sensing zone. The apparatus further includes 1) electrical circuitry connected to the output of the device or system for producing, upon sensing each particle pulse above a predetermined threshold level, an output pulse having a clearly defined duration established between (a) a clearly defined point on the leading edge of the particle pulse occurring at the time of inflection of the slope of the leading edge of the particle pulse and (b) a clearly defined point on the trailing edge of the particle pulse occurring at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and (2) a device for measuring the duty factor of a train of the output pulses. It is to be understood that duty factor, as used herein, is the total duration of all the output pulses in a given interval of time divided by that interval.

The method includes, for each particle pulse, the steps of: initiating the output pulse at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse, terminating the output pulse at a time substantially equal to the time when the amplitude level of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, and measuring the duration of each output pulse.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph illustrating the typical wave shape of a particle pulse produced by a particle analyzing device upon sensing a particle passing through a sensing zone;

FIG. 2 is a block diagram of the electrical circuits of the apparatus of the present invention for producing a desired output pulse for each particle pulse;

FIG. 3A is a diagram consisting of a series of graphs all on the same time scale illustrating various wave shapes of signals or pulses at different points in the electrical circuits shown in FIG. 2;

FIG. 3B is a continuation of the diagram shown in FIG. 3A;

FIG. 4 is a diagram showing the second derivative of a particle pulse being combined with the output of a low threshold circuit; and

FIG. 5 is a diagram of superimposed wave forms of two signals which are compared by a comparator for initiating a control pulse for terminating the output pulse produced by the apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in greater detail, a pulse is shown in FIG. I and is characteristic of a particle pulse produced by a Coulter particle analyzing devicewhen a particle travels through the sensing zone of the Coulter device. As shown, the pulse 10 has a leading edge 12 and a trailing edge 14 and the beginning of the leading edge 12 and the trailing edge 14 of the pulse 10 are gradual and not clearly defined. Accordingly, it is difficult to ascertain the exact point in time when the pulse 10 starts and ends. Consequently, when it is desired to measure the duration of each particle in the sensing zone, it has been found more practical to measure a portion of each particle pulse than to measure the beginning and end of each particle pulse, each portion starting at some point on the leading edge of the particle pulse and ending at some point on the trailing edge of the particle pulse. According to the teachings of the present invention, a method and apparatus are provided for producing an output pulse which begins at a point of inflection 16 of the slope of the leading edge 12 of the particle pulse 10 and ends at a point 18 on the trailing edge of the particle pulse 10, the point 18 being at an instantaneous value on the trailing edge 14 of the particle pulse 10 substantially equal to the instantaneous value at the point of inflection 16 of the slope of the leading edge 12 of the particle pulse 10 as stored in an analog memory. The point of inflection 16 occurs at a time t and the point 18 on the trailing edge 14 when the instantaneous value of the trailing edge 14 of the pulse 10 subsides or falls below the instantaneous value at the point of inflection 16 occurs at a time 1 The output pulse produced by the method and apparatus of the present invention therefore will have a duration equal to the time interval between t and t Typically, the point of inflection 16 occurs at approximately one-half of the amplitude or maximum value of the pulse 10. As will be explained more fully hereinafter, the point of inflection 16 is ascertained by taking the second derivative of the particle pulse, the second derivative signal having a first zero crossing at a point in time substantially equal to the time of the point of inflection 16. According to the teachings of the present invention, this zero crossing is used to initiate a rectangular output pulse having a well-defined leading edge and a predetermined amplitude. In this respect, each particle pulse will produce a constant amplitude output pulse irrespective of the amplitude of the particle pulse.

At the same time the output pulse is being initiated, a portion of the apparatus of the present invention is tracking the input particle pulse and is triggered by the first zero crossing of the second derivative signal to hold the instantaneous value being tracked by the portion of the apparatus thereby to catch and hold an instantaneous value which will always be at substantially the same fraction of the amplitude of the particle pulse independent of the actual amplitude of the particle pulse.

As will be described hereinafter in detail, the instantaneous value held in memory is compared with the instantaneous value of the original particle pulse and at the time when the instantaneous value of the trailing edge of the original particle pulse equals or falls below the held instantaneous value the apparatus of the present invention terminates the output pulse. As stated above, the time of termination is at and the period or interval of the output pulse is t t This time period will be reasonably constant for all particles that travel independently through the aperture, and as stated above, the period or interval of each output pulse will be independent of the amplitude of any particle pulse.

To obtain a measurement of the concentration of particles suspended in a fluid which passes through the aperture of a Coulter particle analyzing device, the output pulses from the apparatus of the present invention are applied to an integrator typically of the type including a capacitor. In this way, each time a particle pulse is developed by a particle passing through the aperture, a unit of charge is produced by the apparatus of the present invention and stored in the integrator. Then, for a given period of time the stored charge in the integrator will be proportional to the total duration of all output pulses which occurred during that given time period. As demonstrated in U.S. Pat. No. 2,775,159 referred to above, this measurement will be closely proportional to the concentration of particles in the scanned fluid. A long sensing zone is used in this patent and the present invention provides a method and apparatus which allow a much smaller sensing zone to be used.

It will be appreciated that if the flow rate changes, the duration of each particle pulse will change and for a fixed concentration of particles the number of particle pulses occurring in a given time period will change. The time period or duration of each output pulse will also change and, therefore, the unit of charge due to each particle pulse will change. It is to be appreciated that longer output pulses which produce larger units of charge occur less frequently since they are due to a slower flow rate. Thus, the duty factor of the output pulse train and, hence, the ultimate duty factor value measured and displayed will be the same. Stated in another way, the narrower output pulses will produce smaller units of charge but more frequently thus holding the total amount of charge applied to the integrator constant.ln this way the present invention provides an apparatus which will measure particle concentration independent of size and flow rate.

It is to be noted that the point of inflection 18 of the slope of the trailing edge 14 of the particle pulse may not necessarily be at the instantaneous value of the point of inflection of the slope of the leading edge of the particle pulse. In this respect and as shown in FIG. 1, the point of inflection of the trailing edge 14 may take place at a point which is ahead in time with respect to the point 18 or at a point 22 which is later in time with respect to the point 18. Also, it is to be noted that not all particle pulses generated by particles flowing through the aperture in the Coulter particle analyzing device will have the characteristic bell shape of the particle pulse 10. In this respect, when two particles pass through the aperture at almost the same point in time, the resulting pulse generated by the two particles will have any one of a variety of shapes. If the two particles are coincident or essentially in line with each other, the apparatus of the present invention will essentially sense only one particle. The occurrence of perfectly simultaneous travel of two particles through the aperture is very remote. Not too infrequently, however, two particles will pass through the aperture very close together such that the resulting particle pulse is a pulse having two peaks and a valley therebetween. If the amplitude at the bottom of the valley is above the instantaneous value at the point of inflection on the slope of the leading edge of the particle pulse, the apparatus of the present invention will only produce one output pulse which, however, will have a greater period or interval than a pulse generated by one particle. On the other hand, if the instantaneous value at the bottom of the valley is less than the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge the apparatus of the present invention will produce two discrete pulses.

Particles which flow through the aperture of the Coulter particle analyzing device along a nonaxial path will not produce a pulse having the bell shape. Such a nonuniform or nonsymmetrical particle pulse will not, however, adversely affect the operation of the apparatus of the present invention or the ability of the apparatus of the present invention to provide an output pulse having a predetermined amplitude and a time period or duration directly related to the time the particle is traveling through the aperture, since the probability of nonaxial flow is the same for all particle systems and the effect will be merely a slight shift in calibration.

Turning now to the other Figures, the apparatus of the present invention is shown schematically in FIG. 2. The apparatus includes an electrical system or particle analyzing device 24 which is typically a Coulter particle analyzing device of the type disclosed in US. Pat. No. 3,259,842 issued on July 5, 1966 to W. H. Coulter et al. The device 24 includes a current source 26 and a scanner 28. Although not shown, it will be understood that the scanner 28 includes a conventional so-called Coulter stand, in which two bodies of electrolyte are in communication with each other through a very small aperture. Means are provided for causing liquid flow of the electrolyte from one body to the other body through the aperture, and two electrodes are connected to the current source 26 and disposed on either side of the aperture for the purpose of establishing a current through the aperture. When a particle flows through the aperture, the impedance therein is changed and a signal a particle pulse is generated. Such a particle pulse is shown at 10a in FIG. 2. It will be noted that in the illustrated embodiment, the pulse 10a starts at some minus value and has a peak amplitude at some plus value. This is because the baseline of the pulse is not at zero and can be at a plus or minus value. Also, the pulse 10a is identical to the pulse 10 except for the position of its baseline.

In addition to the device 24, the apparatus of the present invention can be broken down into two major circuit portions, namely, a first circuit 31 which generates a first control signal at a first point in time for initiating a desired output signal, and a second circuit 32 which generates a second control signal at a second point in time for terminating the output signal. The device 24 is connected via leads 33 and 34 to the first and second circuits 31 and 32 for applying each particle pulse 10a to the circuits 31 and 32.

Briefly, the first circuit 31 includes a circuit 36 for taking the second derivative of the particle pulse 10a. The second derivative circuit 36 has an input connected to the lead 34 and an output connected to one input 37 of a summing network 38. The output of a low threshold circuit 40 comprising a comparator 40a and an adjustable voltage source 40b is connected to another input 41 of the summing network 38 and an output 42 of the summing network 38 is connected to the inverting input 43 of a comparator 44. The leading edge of the output signal from the comparator 44 is detected by a leading edge detector 44a and routed to a 7 clock input 45 of a signal generating circuit or first flipflop 46. The first flip-flop 46 has a Q output 50 connected via leads 51 and 52 through a leading edge detector 53 and a single shot 53a to reset input54 of a control circuit or second flip-flop 56. The first controlsignal appears at the Q output 50.

Briefly, the second circuit 32 includes a clamp and delay circuit 58 (which, being connected to the low threshold circuit 40, can be considered as also forming a part of the first circuit 31) having an input connected to the lead 33, a signal or pulse comparing circuit 62 connected to output 63 of the clamp and delay circuit 58, and an AND circuit 66 having one input terminal 67 connected to output 68 of the signal comparing circuit 62 and another input terminal 69 connected via a lead 70 and the lead 51 to the Q output 50 of the first flip-flop 46. Output terminal 72 of the AND circuit 66 is connected via a lead 74, through a leading edge detector 75 and a one-shot 75a, to set input 76 of the second flip-flop 56. The second control signal appears at the output terminal 72 of the AND circuit 66.

As will be described more fully, hereinafter, the first control signal from the first flip-flop 46 is applied to the reset input 54 of the second flip-flop 56 at substantially the time t, and initiates the desired output pulse from NOT Q (6) output 78 of the second flip-flop 56 which is applied via a lead 80 to an integrator 82. This output pulse is terminated when the second control signal from the output terminal 72 of the AND circuit 66 is applied at substantially time t via the lead 74 to the set input 76 of the second flip-flop 56.

To facilitate a detailed description of the electrical apparatus of the present invention and the operation thereof, the waveforms of the pulses or signals at various points in the apparatus are shown in small scale in FIG. 2 closely adjacent the points at which they occur. These signals and/or pulses are shown in larger scale in FIGS. 3A and 3B. Reference will be made to these wafeforms of signals and pulses in the following detailed description of the components of the apparatus of the present invention and their function.

Turning now to the first circuit 31, the second derivative circuit 36 includes first and second differentiators 84 and 86 and a low pass filter circuit 88. Since the output signals from the differentiators 84 and 86 are somewhat attenuated, the second derivative circuit 36 also includes two amplifiers 90 and 92, the amplifier 90 being connected between the output of the first differentiator 84 and the input of the second differentiator 86 and the amplifier 92 being connected between the output of the second differentiator 86 and the low pass filter circuit 88. Additionally, the second derivative circuit 36 includes a buffer amplifier 94 of unity gain connected between the output of the low pass filter circuit 88 and the input 37 of the summing network 38.

Preferably, the differentiators 84 and 86 are capacitor-resistor networks and the amplifiers 90 and 92 are linear amplifiers with a voltage gain of and a nominal output voltage of zero for no signal input. The low pass filter circuit 88 is preferably a four pole filter which removes high frequency noise from the second derivative signal. The unity gain amplifier 94 is used to isolate the low pass filter circuit 88 from the summing network 38 and the low threshold circuit 40.

As shown, the particle pulse 10a is applied to the input of the differentiator 84 to obtain a first derivative signal 95 which is best shown in graph B of FIG. 3A. (In

the following description, the graph of FIG. 3A or 3B in which the signal or pulse is shown in larger scale, will be referred to merely as graph A, graph B, etc. without reference to FIG. 3A or 3B). The first derivative signal is amplified by the amplifier 90 to provide an amplified first derivative signal 97 (graph C) which is differentiated by the second differentiator 86 to obtain a second derivative signal 99 (graph D) which is amplified by the amplifier 92 to obtain an amplified second derivative signal 101 (graph E) Parenthetically, it is to be noted from graphs A-E that the apparent starting point of the pulse 10 in graph A is at one time whereas the amplified first derivative signal 97 appears to start at an earlier time and the amplified second derivative signal 99 appears to start at an even earlier time. This phenomenon is occasioned by the fact that the beginning and end of the pulse 10 (10a) are not well defined. Thus, when the pulse 10a is differentiated and amplified, the beginning and end of the pulse 10a are more clearly discernible and the pulse duration appears to be expanded.

The amplified second derivative signal 101 is then passed through the filter circuit 88 to obtain a second derivative signal 103 (graph F) with reduced noise. The filtering, however, occasions a delay in time between the signals 101 and 103 as shown at D in graph F.

Although the clamp and delay circuit 58 has been characterized as part of the second circuit 32, it is to be understood that it also forms part of the first circuit 31 in that the output from the clamp and delay circuit 58 is applied to the low threshold circuit 40. The clamp and delay circuit 58 includes a clamp circuit or D. C. restorer circuit 106 and first and second delay circuits 108 and 110 connected through a buffer amplifier 112. Typically, the delay circuits are resistor-capacitor circuits. The clamp circuit 106 is utilized to clamp the pulse 10a to the common potential or ground of the electrical apparatus to produce the pulse 10 which is clamped to the zero baseline at the output of the clamp circuit 106. The pulse 10 is then applied to the first delay circuit 108 where it is delayed by a time D, as shown in graph G to produce the first delayed pulse 113 (graph G). The delayed pulse 113 is buffered by unity gain amplifier 112 connected between the delay circuit 108 and the delay circuit 110. In the second delay circuit 110, the delayed particle pulse 113 is delayed again as shown at D in graph H to provide a second delayed particle pulse 117 (graph H) which is delayed a time D, D with respect to the particle pulse 10. This delay (D, D compensates for the delay incurred in the low pass filter circuit 88 and in other logic circuitry of the apparatus of the present invention.

The low threshold circuit 40 includes the comparator 40a which has an adjustable fixed bias shown at 121 in FIG. 2 applied from the voltage source 40b to the noninverting input 122 thereof. The output 63 of the clamp and delay circuit 58 is connected via a lead 123 to the inverting input 124 of the comparator 40a. The quiescent output signal from the comparator 40a is a logic one signal and is shown at 129 in graph I of FIG. 3A. This logic one output signal from comparator 40a changes to logic zero when the instantaneous value of the leading edge of the clamped and delayed pulse 117 rises above the threshold bias voltage 121 applied to the non-inverting input 122 of the comparator 40a. Also, the logic output from the comparator 400 will change from logic zero to logic one when the instantaneous value of the trailing edge of the clamped and delayed particle pulse 117 falls below the threshold level of the bias voltage 121.

The output signal 129 is applied to the input 41 of the summing network 38 which as shown includes at least two resistors. In this way, the delayed second derivative signal 103 is added to the logic output signal 129 from the comparator 40a in the summing network 38 to obtain a combined signal or trigger signal 131 (graph J) which has a specific first zero crossing at time This combining of the two signals 103 and 129 is best shown in FIG. 4. It will be noted that the first zero crossing of the first trigger signal 131 is at the point of inflection 16 of the leading edge of the particle pulse delayed by the time D.

This trigger signal 131, with a first zero crossing at time 1 is applied to the inverting input 43 of the comparator 44 which functions as a zero crossing detector and which is preferably a high gain open loop amplifier with digital logic output voltage levels. As shown, the comparator 44 also has a non-inverting input 132 connected to the common potential or ground for the apparatus. The comparator 44 has a quiescent output signal of logic zero and is adapted, when the trigger input signal 131 applied to the inverting input 43 thereof makes its first zero crossing, to produce a logic one output signal. The logic output signal from the comparator 44 is shown at 135 in graph K and can be referred to as an intermediate control signal.

It will be apparent from the foregoing description that any time the input signal to the inverting input 43 of the comparator 44 is close to or less than zero, the comparator 44 will produce a logic one output signal. Since it is desired that a logic one output signal only be produced at the time of the first zero crossing of the trigger signal 131, some means must be provided to prevent extraneous negative low amplitude signals from being applied to the inverting input 43 of the comparator 44. It is for this reason that the low threshold circuit 40 comprising principally the comparator 40a is provided in the apparatus of the present invention. In this respect the low threshold circuit 40 and its associated voltage sources and dividers applies a logic one input signal to the inverting input 43 of the comparator 44 until the second derivative input pulse 103, also applied to the comparator 44 through the summing network 38 is at an amplitude level distinctly above zero. Thus, as best shown in FIG. 4, the leading edge of the second derivative signal 103 is added to the logic one signal 129 from the low threshold circuit 40 and, when the instantaneous value of a particle pulse 10 exceeds the predetermined threshold level established by bias voltage 121, the low threshold circuit 40 produces a logic zero signal which is added to the mid-portion of the second derivative signal 103 whereby a trigger signal 131 has a specific first zero crossing which is applied to the inverting input 43 of the comparator 44 and substantially all extraneous negative amplitude signals are prevented from actuating the comparator 44.

It is to be noted that as an alternative to the low threshold circuit 40, the second derivative signal 103 can be clamped to some positive voltage so that the leading and trailing edges thereof would be above zero amplitude. However, such an alternative subcircuit would further ofiset the point in time of the first zero crossing of the second derivative signal and some means for compensating for this change in time would have to be introduced into the apparatus of the present invention.

The low threshold circuit also serves as a threshold control which prevents operation of the apparatus of the present invention when the amplitude of a particle pulse 10 (10a) is below the threshold level established by bias voltage 121. It will be readily apparent that the sensitivity of the apparatus or control over the minimum size of the particle to be counted, can be easily controlled by adjusting the level of the bias voltage 121.

Continuing with the description of circuit 31, the leading edge detector 44a is interposed in the connection between the output of the comparator 44 and the clock input 45 of the first flip-flop 46 so that only the leading edge of the intermediate control signal is applied to the clock input 45. In this respect, the input signal to the clock input of the first flip-flop 46 is a narrow pulse as shown at 139 in graph L of FIG. 3B.

As shown, the flip-flop 46 has a data input 140, to which is applied a logic one data signal indicated at 141 in FIG. 2, and a reset input 142. The first flip-flop 46 is adapted upon receiving the input pulse 139 at the clock input 45 to transfer the data signal 141 at the data input to the Q output 50. Thus, when the pulse 139 is applied to the clock input 45, the logic output signal from the Q output 50 of the first flip-flop 46 changes from logic zero to logic one. This output signal from the Q output 50 of the first flip-flop 46 is indicated at 143 in graph M of FIG. 3B and can be referred to as the first control signal produced by the first circuit 31. The first control signal 143 is applied via leads 51 and 52 to the reset input 54 of the second flip-flop 56 through the leading edge detector 53 and the singleshot or univibrator 53a. The leading edge detector 53 is actuated by the leading edge of the control signal 143 to produce a pulse of short duration which is used to trigger the single-shot 53a which also produces a pulse of short, but predetermined, duration. Since the pulses produced by the leading edge detector 53 and the single-shot 53a are almost the same, they are both represented by the pulse 149 shown in graph S of FIG. 3B.

The quiescent output from the 6 output 78 of the second flip-flop 56 is normally a logic zero output signal and changes from logic zero to logic one when a trigger pulse, such as the pulse 149, is applied to the reset input 54 of the flip-flop 56. Thus, when the pulse 149 is applied to the reset input 54 of the second flipflop 56, the output signal from the 6 output 78 changes from logic zero to logic one. The logic output signal from the Q output 78 is shown at 151 in graph V and is changed again by a control signal from the second circuit 32 as will be described hereinafter in detail, so that the signal 151 is a well-defined output pulse having a predetermined amplitude A, a well-defined leading edge 152 at time t, and trailing edge 153 at time t and a pulse duration or interval t 2 directly proportional to the time a particle is being sensed in the sensing zone. Thus, each time a particle above a predetermined size passes through the aperture in the Coulter particle analyzing device 24, an output pulse 151 is applied to the integrator 82.

Turning now to the second circuit 32, it is apparent that the signal comparing circuit 62 includes several subcircuits, namely, a bypass line 154, a signal track and hold circuit 156, a comparator 158 having an inverting input 159 and a non-inverting input 160, and a switch control circuit 162. The bypass line 154 includes a unity gain amplifier 164 and is connected between the output 63 of the clamp and delay circuit 58 and the inverting input 159 of the comparator 158. The amplifier 164 serves as a buffer between the clamp and delay circuit 58 and the inverting input 159 of the comparator 158. The track and hold circuit 156 is connected between the output 63 of the clamp and delay circuit 58 and the non-inverting input 160 of the comparator 158.

The track and hold circuit 156 includes a unity gain amplifier 166 connected to the output 63 of the clamp and delay circuit 58, an analog switch 168 connected to the output of the amplifier 166 which serves as a buffer between the output 63 and the analog switch 168, a capacitor storage device 170 connected to the output of the analog switch 168, and a second unity gain amplifier 172 connected between the output of the storage device 170 and the non-inverting input 160 of the comparator 158 and serving as a buffer between the track and hold circuit 156 and the comparator 158. The analog switch 168 has a control input 174 and the switch control circuit 162 is connected between the lead 70 from the Q output 50 of the first flip-flop 46 and the control input 174 of the analog switch 168. In the illustrated embodiment, the switch control circuit 162 includes a comparator 178 having a non-inverting input 180 connected to the lead 70, an inverting input 182 which has a fixed positive bias voltage applied thereto as indicated at 183 from an adjustable potentiometer 184 and a logic signal output 186 connected to the control input 174 of the analog switch 168.

During the operation of the signal comparing circuit 62, the analog switch 168 normally presents a low impedance to a signal applied thereto from the amplifier 166 until the analog switch is operated by a switch control signal from the switch control circuit 162 which provides such switch control signal when the first control signal 143 is applied via lead 70 to the noninverting input 180 of the comparator 178 of the switch control circuit 162. At that time the logic output from the switch control circuit 162 changes from a large negative voltage to a large positive voltage as shown at 189 in graph N. The switch control signal 189 is applied to the control input 174 of the analog switch 168 to cause the same to go from the low impedance which passes the clamped and delayed particle pulse 117 to a high impedance which blocks the passage of the pulse 117. It will be understood that part of the pulse 117 has already been passed by the analog switch 168 when the switch control signal from the switch control circuit 162 is applied thereto. Specifically, the analog switch 168 is changed from a low impedance path to a high impedance path at the point in time where the first zero crossing of the first trigger signal 131 occurs, this being substantially equal to the time t, of inflection of the slope of the leading edge 12 of the particle pulse 10. In the meantime, the storage device 170 is tracking the particle pulse 117 passed through the analog switch 168 and is adapted to hold any given instantaneous value passing through the analog switch 168 when it is opened by the switch control signal 189 and will hold that instantaneous value until the analog switch 168 is again closed. This tracked and held signal is shown at 193 in graph and is passed through the unity gain amplifier 172 and applied to the non-inverting input 160 v of the comparator 158. At the same time, the clamped and delayed pulse 117 is applied to the inverting input 159 of the comparator 158.

The quiescent output signal from the comparator 158 is a logic one output signal and changes from logic one to logic zero whenever the input signal to the inverting input 159 is greater than the input signal to the noninverting input 160. Thus, when the clamped and delayed pulse 117 applied to the inverting input 159 is greater than the tracked and held signal or pulse 193 applied to the non-inverting input 160, the logic output signal from the comparator 158 is logic zero, as shown at 195 in graph P. The output signal 195 from the comparator 158 will remain at logic zero until the instantaneous value of the clamped and delayed pulse 117 falls below the value of the signal 193 held by the storage device 170. At that time, the logic output signal 195 changes from logic zero to logic one.

The relationship between the input signals to the inverting input 159 and non-inverting input 160 of the comparator 158 is best shown in FIG. 5 where the clamped and delayed pulse 117 (graph H) is superimposed on the tracked and held pulse 193 (graph 0). Under ideal conditions, the leading and trailing edges of the pulses 117 and 193 should be coincident. However, and as shown in exaggerated form in FIG. 5, the clamped and delayed pulse 117 applied to the inverting input 159 is advantageously ahead in time with respect to the tracked and held pulse 193. As a result, the clamped and delayed pulse 1 17 has a value greater than the tracked and held pulse 193 until the value of the trailing edge of the clamped and delayed pulse 1 17 falls below the value of the tracked and held pulse 193. This slight time delay is incurred in the amplifiers 166, 172, the analog switch 168, and the storage device 170. Thus, as soon as the clamped and delayed pulse 117 is applied to the inverting input 159, the logic output signal from the comparator 158 changes from logic one to logic zero and by reason of the pulse 117 being ahead in time, this change occurs before the time of inflection 16 of the slope of the leading edge 12, the particle pulse 10. Consequently, at the time the output signal 143 from the Q output is applied via the lead to one input terminal 69 of the AND circuit 66, a logic zero signal is being applied to the other input terminal 67 of the AND circuit 66 and the logic output signal from the AND circuit 66 remains at logic zero through the time t of the point of inflection 16.

Then, when the trailing edge of the clamped and delayed pulse 117 falls below the instantaneous value of the tracked and held pulse 193, the logic output signal applied from the comparator 158 to the input terminal 67 changes from logic zero to logic one. This logic one signal 195 can be referred to as an intermediate control signal. With the logic one signal 143 from the Q output 50 still being applied to the other input terminal 69 of the AND circuit, two logic one inputs are now applied to the AND circuit 66. As a result the AND circuit 66 produces a logic one output signal shown at 197 in graph Q of FIG. 3B. This logic one output signal 197 from the AND circuit 66 is of short duration and appears as a pulse and is used to terminate the logic one input signal 143 to the input terminal 69 of the AND circuit 66. The signal 197 constitutes the second control signal which is applied via the lead 74 through the leading edge detector and a single-shot or univibrator 75a to the set input 76 of the second flip-flop 56. The leading edge detector 75 ensures that only the leading edge of the signal 197 will be applied to the single-shot 200; and the single-shot 75a provides a pulse of predetermined duration which is applied to the set input 76. Since the output pulses from the leading edge detector 75 and from the single-shot 75a are almost the same, they are both indicated at 203 in graph R.

The pulse 203 applied to the set input 76 causes the flip-flop 56 to change the output signal from the 60mput 78 from logic one to logic zero thereby, terminating the output pulse 151 and providing the output pulse 151 with the trailing edge 153 substantially at the same point in time when the amplitude of the trailing edge of the clamped and delayed particle pulse 117 falls below the instantaneous value of the leading edge thereof at the point of inflection 16 of the leading edge of the particle pulse 10.

The flip-flop 56 also has a Q output 204 and the quiescent output signal from the Q output 204 is normally logic one. This logic signal from the Q output 204 is shown at 205 in graph T and changes to logic zero when the flip-flop 56 receives the pulse 149 at the reset input 54 and remains at logic zero until the flip-flop 56 receives the signal or pulse 203 at the set input 76. This change from logic zero to logic one is sensed by a trailing edge detector 206 connected to the Q output 204. The trailing edge detector 206 then produces a pulse shown at 209 in graph U which triggers a single-shot or univibrator 210 connected to the output of the trailing edge detector 206. The single-shot produces a pulse similar to the pulse 209 but of predetermined duration. Since the output pulses from the trailing edge detector 206 and the single-shot 210 are almost the same, they are both indicated at 209 in graph U. The pulse 209 is applied via a lead 212 to the reset input 142 of the first flip-flop 46. Upon receiving the pulse 209 at the reset input thereto, the flip-flop 46 changes the logic output signal from the Q output 50 to logic zero. This change in the output signal 143 is not sensed at the reset input 54 of the second flip-flop 56 since the leading edge detector 53 will not sense the trailing edge of the signal 143. However, the change from logic one to logic zero of the signal 143 causes the AND circuit 66 to tenninate the logic one output therefrom such that the logic output signal from the AND circuit 66 is of very short duration. The change in the logic output signal 143 from logic one to logic zero also causes the control circuit 162 to terminate the switch control signal 189 as indicated by the trailing edge of the trigger signal 189 in graph N to close the analog switch 168. The storage device 170 now tracks the trailing edge of the clamped and delayed pulse 117 causing the output signal 193 from the storage device 170 to have the generally truncated shape shown in graph of FIG. 3B. Thus, the second control signal 197, functions not only to form the trailing edge 153 of the pulse 151 from the 6 output 78 at substantially the same time as the amplitude of the trailing edge 14 of the particle pulse 10 falls below the instantaneous value of the leading edge 12 of the particle pulse at the point of inflection 16 of the slope of the leading edge 12 of the particle pulse 10, but also serves to reset the first flip-flop 46 for receiving another intermediate control signal 139 and to terminate the first control signal 143 from the first flipflop 46, which, in turn, terminates the second control signal 197 (by terminating the logic one input to the input terminal 69 of the AND circuit 66) thereby resetting the second circuit 32.

From the foregoing description, it will be understood that the flip-flop 46 transfers the data input 141 to the Q output 50 to provide the desired output signal 143 upon receiving the signal 139 and the duration of the output signal 143 is not dependent upon the duration of the intermediate control signal 135 from the comparator 44 since flip-flop 46 is only actuated by the leading edge of signal 135. In this respect and referring to graph K of FIG. 3A, the second zero crossing of the signal 131 could be at an earlier point in time such that the trailing edge occurs at an earlier point in time as shown by dashed lines at 220 or at a later point in time as shown by dashed lines at 222. In either event, the control signal 143 is not affected by the time of inflection of the slope of the trailing edge of the particle pulse 10. It is only terminated upon the instantaneous value of the trailing edge of the clamped and delayed particle pulse 1 l7 falling below the instantaneous value at the point of inflection 16 of the slope of the leading edge 12 of the particle pulse 10 as held by the track and hold circuit 156.

In order to provide a read out of the information stored in the integrator 82, i.e., the voltage on output 300 of the integrator 82, a readout device 301, which may be a DArsonval meter movement or any other convenient mechanism, is connected to the output 300. Moreover, in order to obtain readings which do not fluctuate with time in a random fashion, it has been found useful to provide a timer 302 which is connected to the integrator 82 by means of connections 303 and 304. When it is desired to take a reading of particle concentration, a push botton switch 305 is depressed, shorting connection 304 to ground, thereby initiating the timing cycle and resetting the integrator 82. The timer 302 enables the integrator 82 by completing the circuit to its input by means of a built-in conventional electronic switch (not shown) and permits the integrator 82 to start to accumulate a charge as described above. At the end of a predetermined time, the integrator 82 is once again disconnected, causing it to hold whatever charge it accumulated during the measurement period. The readout device 301 then makes this amount of charge, which appears in the form of a voltage, usuable for a human or machine operator.

An alternative form of readout can be obtained by substituting a low pass amplifier for the integrator, or by placing a resistance across the integrator capacitor, so that the readout is a measure of the average charge per unit time which is fed into it. In this case, the timer 302 and starting switch 305 are not required, although the reading may fluctuate more or less depending upon the time constant of the resulting combination.

Although a preferred embodiment of the apparatus of the present invention has been described, it will be understood that obvious modifications and variations can be made to the apparatus and method without departing from the spirit or scope of the invention. Accordingly, the scope of the present invention is only to be limited as necessitated by the accompanying claims.

What it is desired to secure by Letters Patent of the United States is:

1. Apparatus for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said apparatus including A. means connected to the output of the electrical system for producing upon sensing each particle pulse above a given threshold level an output pulse having a given amplitude and a selected clearly defined duration established therebetween i. a clearly defined leading edge of said output pulse occurring at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse and ii. a clearly defined trailing edge of said output pulse occurring at substantially the same time that the instantaneous value of the particle pulse at its trailing edge falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and

B. means for measuring and displaying the duty factor of said output pulses to obtain a measurement of the concentration of particles in the fluid traversing the sensing zone.

2. The apparatus as claimed in claim 1 wherein said means for producing said output pulse includes means for initiating said output pulse at a point in time substantially equal to the time when there is an inflection of the slope of the leading edge of the particle pulse.

3. The apparatus as claimed in claim 2 wherein said means for initiating said output pulse includes means for taking the second derivative of the particle pulse to provide a second derivative signal which has a first zero crossing at the point of inflection of the slope of the leading edge of the particle pulse.

4. The apparatus as claimed in claim 3 wherein said means for initiating said output pulse includes means for amplifying said second derivative signal and low pass filter means for reducing the signal to noise ratio of the amplified second derivative signal.

5. The apparatus as claimed in claim 4 including low threshold circuit means connected to said electrical system for detecting a threshold amplitude level of the particle pulse, said low threshold circuit means being adapted to prevent operation of said apparatus if the amplitude level of the particle pulse does not exceed a predetermined threshold level.

6. The apparatus as claimed in claim 5 including means for delaying the time of application of the particle pulse to said low threshold circuit means to compensate for the time delay in said means for taking the second derivative of the particle pulse occasioned by the amplification and filtering of said second derivative signal.

7. The apparatus as claimed in claim 3 including low threshold circuit means connected to said electrical system for preventing operation of said apparatus if the amplitude of the particle pulse does not exceed a predetermined threshold level.

8. The apparatus as claimed in claim 7 including means for clamping the particle pulse applied to said low threshold circuit to a common potential for said apparatus.

9. The apparatus as claimed in claim 7 wherein the quiescent output'signal from said low threshold circuit means is a logic one output signal and said low threshold circuit means is adapted to change its logic one output to a logic zero output when the instantaneous value of the particle pulse applied to the low threshold circuit means exceeds the predetermined threshold level and wherein said apparatus includes means for summing said output signal from said low threshold circuit with said second derivative signal whereby the leading edge of said second derivative signal is added to a logic one signal and said first zero crossing of said second derivative signal is added to a logic zero signal to produce a timing signal having a specific first zero crossing.

10. The apparatus as claimed in claim 9 wherein said means for initiating said output pulse includes comparator circuit means connected to said summing means for producing a trigger signal at the time when said timing signal makes its first zero crossing.

11. The apparatus as claimed in claim 1 wherein said means for producing said output pulse includes means for terminating said output pulse when the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.

12. The apparatus as claimed in claim 11 wherein said means for terminating said output pulse includes a signal comparing circuit for sensing the particle pulse, for sensing and holding the instantaneous value of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, for comparing the changing instantaneous values of the particle pulse with the held instantaneous value of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse and for producing a control signal at the time the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value level, said control signal being utilized to terminate said output pulse.

13. The apparatus as claimed in claim 1 wherein said means for producing said well-defined output pulse includes first means for initiating said output pulse at a point in time substantially equal to the time when there is an inflection of the slope of the leading edge of the particle pulse and second means for terminating said output pulse when the instantaneous value of the trailing edge of said particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.

14. The apparatus as claimed in claim 13 wherein said first means for initiating said output pulse includes means for producing a first control signal at substantially the same time when there is a point of inflection of the slope of the leading edge of the particle pulse and said second means for terminating said output pulse in cludes a comparator circuit for comparing the instantaneous value of the particle pulse with the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse and for producing a second control signal at the time when the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse.

15. The apparatus as claimed in claim 14 wherein said second means includes a track and hold circuit for tracking the instantaneous value of the particle pulse, for holding the instantaneous value at the point of inflection of the leading edge of the particle pulse and for applying this held instantaneous value to said comparator circuit.

16. The apparatus as claimed in claim wherein said track and hold circuit has a switch control input and said second means includes a switch control circuit connected between the output of said first means and said switch control input of said track and hold circuit, said switch control circuit being adapted, upon receiving said first control signal from said first means to apply a switch control signal to said track and hold circuit to cause same to hold the level of the instantaneous value of the particle pulse being tracked whereby said track and hold circuit holds an instantaneous value which is substantially equal to the instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse.

17. The apparatus as claimed in claim 13 wherein said first means for initiating said output pulse includes means for taking the second derivative of the particle pulse and for producing a second derivative signal having a first zero crossing at a time substantially equal to the time there is an inflection of the slope of the leading edge of the particle pulse.

18. The apparatus as claimed in claim 17 including low threshold circuit means connected to said electrical system and adapted to sense the instantaneous value of the particle pulse and to prevent operation of said apparatus if the instantaneous value of the particle pulse does not exceed a predetermined threshold level.

l9.-The apparatus as claimed in claim 18 wherein said low threshold circuit means produces a quiescent logic one output signal and is adapted to provide a logic zero output signal when said low threshold circuit senses an instantaneous value of the particle pulse above said predetermined threshold level and wherein said apparatus includes summing circuit means for summing said logic output signal from said low threshold circuit with said second derivative signal whereby the leading edge of said second derivative signal is added to a logic one signal and said first zero crossing of said second derivative signal is added to a logic zero signal to produce a control signal having a specific first zero crossing.

20. The apparatus as claimed in claim 19 wherein said first means includes signal comparing means for sensing the first zero crossing of said control signal and for producing an output pulse initiating signal having a well-defined leading edge at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse.

21. The apparatus as claimed in claim 13 wherein said first means includes means for producing a first control signal at substantially the time of inflection of the slope of the leading edge of the particle pulse and said second means includes means for producing a second control signal at the time the amplitude level of the trailing edge of the particle pulse falls below the amplitude level of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, and wherein said apparatus includes control circuit means for producing said output pulse, said control circuit means being connected to said first and second means and being adapted to initiate said output pulse upon receiving said first control signal from said first means and being adapted to terminate said output pulse upon receiving said second control signal from said second means.

22. The apparatus as claimed in claim 21 wherein said second means includes an AND circuit, the quiescent output of which is a logic zero output signal, and means for producing an intermediate control signal which is applied to said AND circuit and which changes from logic one to logic zero at substantially the time of inflection of the slope of the leading edge of the particle pulse and which changes from logic zero to logic one at substantially the time the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, said first control signal from said first means being a logic one signal which is also applied to said AND circuit whereby, when said intermediate control signal changes from logic zero to logic one, the two inputs to said AND circuit are both logic one inputs resulting in a logic one output from said AND circuit which constitutes said second control signal which is applied to said control circuit means for causing said control circuit means to terminate said output pulse.

23. The apparatus as claimed in claim 22 wherein said first means includes means for producing an intermediate control signal at substantially the same time as the point of inflection of the slope of the leading edge of the particle pulse, and a signal generating circuit having a trigger input, a reset input and an output and being adapted to produce said first control signal when said intermediate control signal is applied to said trigger input, said first control signal being applied to said AND circuit and to said control circuit means, and wherein said control circuit means has a second output which is connected to said reset input of said signal generating circuit of said first means, the quiescent output signal from said second output of said control circuit means being a logic one output signal and changing from logic one to logic zero when said first control signal from said signal generating circuit of said first means is applied to said control circuit means and changing from logic zero to logic one when said second control signal from said AND circuit is applied to said control circuit means, the trailing edge of said output signal from said second output of said control circuit means being applied to said reset input of said signal generating circuit to terminate said first control signal, the termination of which terminates said second control signal from said AND circuit to reset said apparatus for sensing another particle pulse.

24. The apparatus as claimed in claim 23 wherein said second means includes signal tracking means for tracking the particle pulse, signal value holding means for holding a given instantaneous value of a signal at a given point in time and switch control means, connected between said signal value holding means and said output of said signal generating circuit, for controlling said signal value holding means to hold a given instantaneous value of the particle pulse upon receiving said first control signal from said signal generating circuit.

25. The apparatus as claimed in claim 1 wherein said means includes first and second input channels for receiving the particle pulse, said first input channel including means for clamping each particle pulse to a common potential and means for delaying the clamped particle pulse, said second input channel including means for producing a signal equal to the second derivative of said particle pulse, said second derivative signal having at least two zero potential crossings corresponding respectively to the points of inflection of the respective slopes of the leading and trailing edges of the particle pulse, and filter means for increasing the signal to noise ratio of the second derivative signal, a low threshold circuit connected to the output of said first input channel and including means for changing a logic one output signal from said low threshold circuit to a logic zero output signal when the instantaneous value of the clamped and delayed particle pulse from said first input channel reaches a predetermined threshold value, summin g circuit means connected to said output of said low threshold circuit and to the output of said second input channel for combining the filtered second derivative signal with the logic output signal from said low threshold circuit whereby the leading edge of said second derivative signal is added to a logic one signal and the first zero crossing of said second derivative signal is added to a logic zero signal to produce a combined second derivative signal having a specific first zero crossing, first circuit means connected to said summing circuit means for receiving the combined second derivative signal and for producing a first intermediate control signal having a clearly defined leading edge at a time substantially equal to the first zero crossing of said second derivative signal, second circuit means connected to said first circuit means for producing, at an output thereof, a first logic one control signal upon sensing the leading edge of said first intermediate control signal, said second circuit means also having a reset input for receiving a reset signal for terminating said first control signal, a triggered, signal comparing circuit including a track and hold circuit connected to said output of said first input channel, a switch control circuit connected between said output of said second circuit means and said track and hold circuit, and a comparator having a first input terminal connected to said output of said first input channel and a second input terminal connected to the output of said track and hold circuit, said track and hold circuit being adapted to track said clamped and delayed particle pulse from said input channel and to hold the instantaneous value of said clamped and delayed particle pulse at the time when said track and hold circuit is placed in its hold mode by a switch control signal from said switch control circuit, said switch control circuit producing said switch control signal upon receiving said first control signal from said second circuit means whereby said switch control signal commences at a point in time substantially equal to the time said second derivative signal of said leading edge of said particle pulse makes said first zero crossing and the instantaneous value held by said track and hold circuit is the instantaneous value at the point of inflection of the leading edge of the particle pulse, the quiescent output signal from said comparator being a logic one signal and said comparator being adapted to produce a logic zero output signal during the time the instantaneous value of said clamped and delayed particle pulse received at said first input terminal of said comparator is greater than the held instantaneous value received at said second input terminal of said comparator from said track and hold circuit, an AND circuit having a first input terminal connected to the output of said comparator and a second input terminal connected to said output of said second circuit means and adapted to produce a second control signal when the output signal from said comparator changes from logic zerovto logic one and said first control signal is applied to said second input terminal of said comparator, a third circuit means for producing said well-defined output pulse, said third circuit means having first and second inputs and first and second outputs, said first input of said third circuit means being connected to said output of said second circuit means, said second input of said third circuit means being connected to the output of said AND .circuit, said second output of said third circuit means being connected to said reset input of said second circuit means, and said means for measuring and displaying the duty factor of said output pulses includes an integrator connected to said first output of said third circuit means for integrating each output pulse received from said third circuit means, means for timing said integrator, means for resetting said integrator, and means for displaying a readout of the output from said integrator, said third circuit means being adapted, upon receiving at its first input said first control signal from said second circuit means, to change the signal at said first output thereof from logic zero to logic one to start one of said output pulses, the leading edge of said output pulse occurring at substantially the same time as the inflection of the slope of the leading edge of the particle pulse, and to change the signal at said second output thereof, which is applied to said reset input of said third circuit means, from logic one to logic zero, and being adapted upon receiving at its second input said second control signal from said AND circuit to change the logic output signal at said first output thereof from logic one to logic zero to terminate said output pulse, the trailing edge of said output pulse from said third circuit means occurring at a time substantially equal to the time the amplitude of the trailing edge of the particle pulse falls below the held instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse and to change the output signal at said second output thereof from logic zero to logic one, the trailing edge of this output signal being applied to said reset input of said second circuit means to cause said second circuit means to terminate said first control signal at the output thereof which in turn terminates said second control signal from said AND circuit to reset said apparatus for receiving another particle pulse, said timing means and said resetting means being operable to said integrator to integrate for a predetermined measuring interval, and said displaying means providing a display of the final charge accumulated and held by said integrator.

26. A method for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said method including, for each particle pulse, the steps of: initiating an output pulse at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse; terminating said output pulse at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse so that each output pulse has a clearly-defined duration; and measuring the duration of each output pulse for the purpose of obtaining the duty factor of the output pulses with respect to time to obtain a measurement of the concentration of particles in the fluid which traverses the sensing zone.

27. The method as claimed in claim 26 wherein said output pulse is initiated by taking the second derivative of the particle pulse to obtain a second derivative signal the first zero crossing of which is at substantially the same time as the inflection of the slope of the leading edge of the particle pulse, and said method includes the step of using this zero crossing of the second derivative signal to produce a control signal which initiates said output pulse.

28. The method as claimed in claim 27 including the steps of adding a logic signal to said second derivative signal, said logic signal consisting of a logic one signal up to a point in time when the instantaneous value of the particle pulse exceeds a predetermined threshold level and being a logic zero signal from this point in time until the trailing edge of the particle pulse falls below the predetermined threshold level whereby the leading and trailing edges of the second derivative signal are added to a logic one signal and the zero crossings of the second derivative signal are added to a logic zero signal to obtain a control signal having a specific first zero crossing at the time of inflection of the slope of the leading edge of the particle pulse.

29. The method as claimed in claim 26 wherein said output pulse is terminated by a control signal, said control signal being obtained by holding the instantaneous value of the leading edge of the particle pulse at the point of inflection of the leading edge of the particle pulse, by comparing this held instantaneous value with the changing instantaneous value of the particle pulse and by initiating said control signal when the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.

30. A method for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said method including, for each particle pulse, the steps of: sensing the particle pulse; taking the second derivative of the particle pulse to obtain a second derivative signal having a zero crossing at the point of inflection of the slope of the leading edge of the particle pulse; using this zero crossing of the second derivative signal to produce a first control signal; using the first control signal to initiate an output pulse with a well-defined leading edge at substantially the time of inflection of the slope of the leading edge of the particle pulse; tracking the particle pulse with a track and hold circuit; holding the instantaneous value of the tracked pulse at the point of inflection of the leading edge of the particle pulse by applying the first control signal to the track and hold circuit; comparing the held instantaneous value in the track and hold circuit with the changing instantaneous value of the particle pulse; initiating a second control signal when the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse; using the second control signal to terminate the output pulse with the output pulse having a clearly defined trailing edge at substantially the time the instantaneous value of the trailing edge of the particle pulse falls below the amplitude level of the leading edge at the time of inflection of the slope of the leading edge of the particle pulse so that each pulse has a clearly defined duration, and integrating each output pulse to obtain a measurement of the particle concentration in said fluid suspension.

31. The method as defined in claim 29 wherein said second control signal is only produced when said first control signal is in existence and said second control signal is used to terminate said first control signal with the termination of said first control signal serving to terminate said second control signal to enable said method to be quickly and accurately repeated for a subsequent particle pulse.

32. The method as claimed in claim 29 including the step of adding a logic signal to said second derivative signal to produce a trigger signal which is used to generate said first control signal, said logic signal consisting of a logic one signal up to a point in time when the instantaneous value of the particle pulse exceeds a predetermined threshold level and being a logic zero signal from this point in time until the trailing edge of the particle pulse falls below the predetermined threshold level whereby the leading and trailing edges of the second derivative signal are added to a logic one signal and the zero crossings of the second derivative signal are added to a logic zero signal to obtain a trigger signal having a specific first zero crossing at the time of the inflection of the slope of the leading edge of the particle pulse.

m UNITED STATES PATENT OFFICE I @ERTIFICATE 0F CQRRECTION Patent N01 3,733,547 Dated May 15, 1973 Inventofls) WALLACE H. COULTER, et al. V

it is certified that error appears in the above-identified patent am? that said Letters Patent: are hereby corrected as shown below:

Column 15, Line 19, Delete "and displaying" Column 20, Line 47, Delete "said" Line 48 Delete 'ldisplaying", after "means" insert --for-.

Signed and sealed this 8th day of January 197M.

(SEAL) Attest:

- EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents 

1. Apparatus for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said apparatus including A. means connected to the output of the electrical system for producing upon sensing each particle pulse above a given threshold level an output pulse having a given amplitude and a selected clearly defined duration established therebetween i. a clearly defined leading edge of said output pulse occurring at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse and ii. a clearly defined trailing edge of said output pulse occurring at substantially the same time that the instantaneous value of the particle pulse at its trailing edge falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and B. means for measuring and displaying the duty factor of said output pulses to obtain a measurement of the concentration of particles in the fluid traversing the sensing zone.
 2. The apparatus as claimed in claim 1 wherein said means for producing said output pulse includes means for initiating said output pulse at a point in time substantially equal to the time when there is an inflection of the slope of the leading edge of the particle pulse.
 3. The apparatus as claimed in claim 2 wherein said means for initiating said output pulse includes means for taking the second derivative of the particle pulse to provide a second derivative signal which has a first zero crossing at the point of inflection of the slope of the leading edge of the particle pulse.
 4. The apparatus as claimed in claim 3 wherein said means for initiating said output pulse includes means for amplifying said second derivative signal and low pass filter means for reducing the signal to noise ratio of the amplified second derivative signal.
 5. The apparatus as claimed in claim 4 including low threshold circuit means connected to said electrical system for detecting a threshold amplitude level of the particle pulse, said low threshold circuit means being adapted to prevent operation of said apparatus if the amplitude level of the particle pulse does not exceed a predetermined threshold level.
 6. The apparatus as claimed in claim 5 including means for delaying the time of application of the particle pulse to said low threshold circuit means to compensate for the time delay in said means for taking the second derivative of the particle pulse occasioned by the amplification and filtering of said second derivative signal.
 7. The apparatus as claimed in claim 3 including low threshold circuit means connected to said electrical system for preventing operation of said apparatus if the amplitude of the particle pulse does not exceed a predetermined threshold level.
 8. The apparatus as claimed in claim 7 including means for clamping the particle pulse applied to said low threshold circuit to a common potential for said apparatus.
 9. The apparatus as claimed in claim 7 wherein the quiescent output signal from said low threshold circuit means is a logic one output signal and said low threshold circuit means is adapted to change its logic one output to a logIc zero output when the instantaneous value of the particle pulse applied to the low threshold circuit means exceeds the predetermined threshold level and wherein said apparatus includes means for summing said output signal from said low threshold circuit with said second derivative signal whereby the leading edge of said second derivative signal is added to a logic one signal and said first zero crossing of said second derivative signal is added to a logic zero signal to produce a timing signal having a specific first zero crossing.
 10. The apparatus as claimed in claim 9 wherein said means for initiating said output pulse includes comparator circuit means connected to said summing means for producing a trigger signal at the time when said timing signal makes its first zero crossing.
 11. The apparatus as claimed in claim 1 wherein said means for producing said output pulse includes means for terminating said output pulse when the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.
 12. The apparatus as claimed in claim 11 wherein said means for terminating said output pulse includes a signal comparing circuit for sensing the particle pulse, for sensing and holding the instantaneous value of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, for comparing the changing instantaneous values of the particle pulse with the held instantaneous value of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse and for producing a control signal at the time the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value level, said control signal being utilized to terminate said output pulse.
 13. The apparatus as claimed in claim 1 wherein said means for producing said well-defined output pulse includes first means for initiating said output pulse at a point in time substantially equal to the time when there is an inflection of the slope of the leading edge of the particle pulse and second means for terminating said output pulse when the instantaneous value of the trailing edge of said particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.
 14. The apparatus as claimed in claim 13 wherein said first means for initiating said output pulse includes means for producing a first control signal at substantially the same time when there is a point of inflection of the slope of the leading edge of the particle pulse and said second means for terminating said output pulse includes a comparator circuit for comparing the instantaneous value of the particle pulse with the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse and for producing a second control signal at the time when the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse.
 15. The apparatus as claimed in claim 14 wherein said second means includes a track and hold circuit for tracking the instantaneous value of the particle pulse, for holding the instantaneous value at the point of inflection of the leading edge of the particle pulse and for applying this held instantaneous value to said comparator circuit.
 16. The apparatus as claimed in claim 15 wherein said track and hold circuit has a switch control input and said second means includes a switch control circuit connected between the output of said first means and said switch control input of said track and hold circuit, said switch control circuit being adapted, upon receiving said first control signal from saiD first means to apply a switch control signal to said track and hold circuit to cause same to hold the level of the instantaneous value of the particle pulse being tracked whereby said track and hold circuit holds an instantaneous value which is substantially equal to the instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse.
 17. The apparatus as claimed in claim 13 wherein said first means for initiating said output pulse includes means for taking the second derivative of the particle pulse and for producing a second derivative signal having a first zero crossing at a time substantially equal to the time there is an inflection of the slope of the leading edge of the particle pulse.
 18. The apparatus as claimed in claim 17 including low threshold circuit means connected to said electrical system and adapted to sense the instantaneous value of the particle pulse and to prevent operation of said apparatus if the instantaneous value of the particle pulse does not exceed a predetermined threshold level.
 19. The apparatus as claimed in claim 18 wherein said low threshold circuit means produces a quiescent logic one output signal and is adapted to provide a logic zero output signal when said low threshold circuit senses an instantaneous value of the particle pulse above said predetermined threshold level and wherein said apparatus includes summing circuit means for summing said logic output signal from said low threshold circuit with said second derivative signal whereby the leading edge of said second derivative signal is added to a logic one signal and said first zero crossing of said second derivative signal is added to a logic zero signal to produce a control signal having a specific first zero crossing.
 20. The apparatus as claimed in claim 19 wherein said first means includes signal comparing means for sensing the first zero crossing of said control signal and for producing an output pulse initiating signal having a well-defined leading edge at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse.
 21. The apparatus as claimed in claim 13 wherein said first means includes means for producing a first control signal at substantially the time of inflection of the slope of the leading edge of the particle pulse and said second means includes means for producing a second control signal at the time the amplitude level of the trailing edge of the particle pulse falls below the amplitude level of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, and wherein said apparatus includes control circuit means for producing said output pulse, said control circuit means being connected to said first and second means and being adapted to initiate said output pulse upon receiving said first control signal from said first means and being adapted to terminate said output pulse upon receiving said second control signal from said second means.
 22. The apparatus as claimed in claim 21 wherein said second means includes an AND circuit, the quiescent output of which is a logic zero output signal, and means for producing an intermediate control signal which is applied to said AND circuit and which changes from logic one to logic zero at substantially the time of inflection of the slope of the leading edge of the particle pulse and which changes from logic zero to logic one at substantially the time the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, said first control signal from said first means being a logic one signal which is also applied to said AND circuit whereby, when said intermediate control signal changes from logic zero to logic one, the two inputs to said AND circuit are both logic one inputs resulting in a logic one oUtput from said AND circuit which constitutes said second control signal which is applied to said control circuit means for causing said control circuit means to terminate said output pulse.
 23. The apparatus as claimed in claim 22 wherein said first means includes means for producing an intermediate control signal at substantially the same time as the point of inflection of the slope of the leading edge of the particle pulse, and a signal generating circuit having a trigger input, a reset input and an output and being adapted to produce said first control signal when said intermediate control signal is applied to said trigger input, said first control signal being applied to said AND circuit and to said control circuit means, and wherein said control circuit means has a second output which is connected to said reset input of said signal generating circuit of said first means, the quiescent output signal from said second output of said control circuit means being a logic one output signal and changing from logic one to logic zero when said first control signal from said signal generating circuit of said first means is applied to said control circuit means and changing from logic zero to logic one when said second control signal from said AND circuit is applied to said control circuit means, the trailing edge of said output signal from said second output of said control circuit means being applied to said reset input of said signal generating circuit to terminate said first control signal, the termination of which terminates said second control signal from said AND circuit to reset said apparatus for sensing another particle pulse.
 24. The apparatus as claimed in claim 23 wherein said second means includes signal tracking means for tracking the particle pulse, signal value holding means for holding a given instantaneous value of a signal at a given point in time and switch control means, connected between said signal value holding means and said output of said signal generating circuit, for controlling said signal value holding means to hold a given instantaneous value of the particle pulse upon receiving said first control signal from said signal generating circuit.
 25. The apparatus as claimed in claim 1 wherein said means includes first and second input channels for receiving the particle pulse, said first input channel including means for clamping each particle pulse to a common potential and means for delaying the clamped particle pulse, said second input channel including means for producing a signal equal to the second derivative of said particle pulse, said second derivative signal having at least two zero potential crossings corresponding respectively to the points of inflection of the respective slopes of the leading and trailing edges of the particle pulse, and filter means for increasing the signal to noise ratio of the second derivative signal, a low threshold circuit connected to the output of said first input channel and including means for changing a logic one output signal from said low threshold circuit to a logic zero output signal when the instantaneous value of the clamped and delayed particle pulse from said first input channel reaches a predetermined threshold value, summing circuit means connected to said output of said low threshold circuit and to the output of said second input channel for combining the filtered second derivative signal with the logic output signal from said low threshold circuit whereby the leading edge of said second derivative signal is added to a logic one signal and the first zero crossing of said second derivative signal is added to a logic zero signal to produce a combined second derivative signal having a specific first zero crossing, first circuit means connected to said summing circuit means for receiving the combined second derivative signal and for producing a first intermediate control signal having a clearly defined leading edge at a time substantially equal to the first zero crossing of said second derIvative signal, second circuit means connected to said first circuit means for producing, at an output thereof, a first logic one control signal upon sensing the leading edge of said first intermediate control signal, said second circuit means also having a reset input for receiving a reset signal for terminating said first control signal, a triggered, signal comparing circuit including a track and hold circuit connected to said output of said first input channel, a switch control circuit connected between said output of said second circuit means and said track and hold circuit, and a comparator having a first input terminal connected to said output of said first input channel and a second input terminal connected to the output of said track and hold circuit, said track and hold circuit being adapted to track said clamped and delayed particle pulse from said input channel and to hold the instantaneous value of said clamped and delayed particle pulse at the time when said track and hold circuit is placed in its hold mode by a switch control signal from said switch control circuit, said switch control circuit producing said switch control signal upon receiving said first control signal from said second circuit means whereby said switch control signal commences at a point in time substantially equal to the time said second derivative signal of said leading edge of said particle pulse makes said first zero crossing and the instantaneous value held by said track and hold circuit is the instantaneous value at the point of inflection of the leading edge of the particle pulse, the quiescent output signal from said comparator being a logic one signal and said comparator being adapted to produce a logic zero output signal during the time the instantaneous value of said clamped and delayed particle pulse received at said first input terminal of said comparator is greater than the held instantaneous value received at said second input terminal of said comparator from said track and hold circuit, an AND circuit having a first input terminal connected to the output of said comparator and a second input terminal connected to said output of said second circuit means and adapted to produce a second control signal when the output signal from said comparator changes from logic zero to logic one and said first control signal is applied to said second input terminal of said comparator, a third circuit means for producing said well-defined output pulse, said third circuit means having first and second inputs and first and second outputs, said first input of said third circuit means being connected to said output of said second circuit means, said second input of said third circuit means being connected to the output of said AND circuit, said second output of said third circuit means being connected to said reset input of said second circuit means, and said means for measuring and displaying the duty factor of said output pulses includes an integrator connected to said first output of said third circuit means for integrating each output pulse received from said third circuit means, means for timing said integrator, means for resetting said integrator, and means for displaying a readout of the output from said integrator, said third circuit means being adapted, upon receiving at its first input said first control signal from said second circuit means, to change the signal at said first output thereof from logic zero to logic one to start one of said output pulses, the leading edge of said output pulse occurring at substantially the same time as the inflection of the slope of the leading edge of the particle pulse, and to change the signal at said second output thereof, which is applied to said reset input of said third circuit means, from logic one to logic zero, and being adapted upon receiving at its second input said second control signal from said AND circuit to change the logic output signal at said first output thereof from logic one to logic zero to terminate said output pulse, the traIling edge of said output pulse from said third circuit means occurring at a time substantially equal to the time the amplitude of the trailing edge of the particle pulse falls below the held instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse and to change the output signal at said second output thereof from logic zero to logic one, the trailing edge of this output signal being applied to said reset input of said second circuit means to cause said second circuit means to terminate said first control signal at the output thereof which in turn terminates said second control signal from said AND circuit to reset said apparatus for receiving another particle pulse, said timing means and said resetting means being operable to said integrator to integrate for a predetermined measuring interval, and said displaying means providing a display of the final charge accumulated and held by said integrator.
 26. A method for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said method including, for each particle pulse, the steps of: initiating an output pulse at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse; terminating said output pulse at substantially the same time that the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse so that each output pulse has a clearly-defined duration; and measuring the duration of each output pulse for the purpose of obtaining the duty factor of the output pulses with respect to time to obtain a measurement of the concentration of particles in the fluid which traverses the sensing zone.
 27. The method as claimed in claim 26 wherein said output pulse is initiated by taking the second derivative of the particle pulse to obtain a second derivative signal the first zero crossing of which is at substantially the same time as the inflection of the slope of the leading edge of the particle pulse, and said method includes the step of using this zero crossing of the second derivative signal to produce a control signal which initiates said output pulse.
 28. The method as claimed in claim 27 including the steps of adding a logic signal to said second derivative signal, said logic signal consisting of a logic one signal up to a point in time when the instantaneous value of the particle pulse exceeds a predetermined threshold level and being a logic zero signal from this point in time until the trailing edge of the particle pulse falls below the predetermined threshold level whereby the leading and trailing edges of the second derivative signal are added to a logic one signal and the zero crossings of the second derivative signal are added to a logic zero signal to obtain a control signal having a specific first zero crossing at the time of inflection of the slope of the leading edge of the particle pulse.
 29. The method as claimed in claim 26 wherein said output pulse is terminated by a control signal, said control signal being obtained by holding the instantaneous value of the leading edge of the particle pulse at the point of inflection of the leading edge of the particle pulse, by comparing this held instantaneous value with the changing instantaneous value of the particle pulse and by initiating said control signal when the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.
 30. A method for measuring particle concentration in a fluid suspensioN of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said method including, for each particle pulse, the steps of: sensing the particle pulse; taking the second derivative of the particle pulse to obtain a second derivative signal having a zero crossing at the point of inflection of the slope of the leading edge of the particle pulse; using this zero crossing of the second derivative signal to produce a first control signal; using the first control signal to initiate an output pulse with a well-defined leading edge at substantially the time of inflection of the slope of the leading edge of the particle pulse; tracking the particle pulse with a track and hold circuit; holding the instantaneous value of the tracked pulse at the point of inflection of the leading edge of the particle pulse by applying the first control signal to the track and hold circuit; comparing the held instantaneous value in the track and hold circuit with the changing instantaneous value of the particle pulse; initiating a second control signal when the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse; using the second control signal to terminate the output pulse with the output pulse having a clearly defined trailing edge at substantially the time the instantaneous value of the trailing edge of the particle pulse falls below the amplitude level of the leading edge at the time of inflection of the slope of the leading edge of the particle pulse so that each pulse has a clearly defined duration, and integrating each output pulse to obtain a measurement of the particle concentration in said fluid suspension.
 31. The method as defined in claim 29 wherein said second control signal is only produced when said first control signal is in existence and said second control signal is used to terminate said first control signal with the termination of said first control signal serving to terminate said second control signal to enable said method to be quickly and accurately repeated for a subsequent particle pulse.
 32. The method as claimed in claim 29 including the step of adding a logic signal to said second derivative signal to produce a trigger signal which is used to generate said first control signal, said logic signal consisting of a logic one signal up to a point in time when the instantaneous value of the particle pulse exceeds a predetermined threshold level and being a logic zero signal from this point in time until the trailing edge of the particle pulse falls below the predetermined threshold level whereby the leading and trailing edges of the second derivative signal are added to a logic one signal and the zero crossings of the second derivative signal are added to a logic zero signal to obtain a trigger signal having a specific first zero crossing at the time of the inflection of the slope of the leading edge of the particle pulse. 